The present invention relates to a boat for carrying semiconductor substrates, which boat is used in an impurity-diffusion process, an oxidation process or an annealing process.
Recently, the size of a semiconductor substrate, for example, a silicon wafer used for producing semiconductor devices, e.g., ICs and LSIs, has become larger than 2 inches in diameter. When a large number of the semiconductor substrates are subjected to a diffusion or oxidation process, the total weight of a boat, which is made of quartz and carries a large number of the large size semiconductor substrates, is heavy. When the heavy boat is put in and out of a quartz tube of a diffusion furnace or an oxidation furnace, finely powdered quartz is generated by friction between the quartz boat and the quartz tube. In the case of a boat made of polycrystalline silicon instead of the quartz, finely powdered quartz and polycrystalline silicon are generated by friction between the polycrystalline silicon boat and the quartz tube. In order to reduce the generation of fine powder to as small an amount as possible, the boat is provided with wheels made of the same material as that of the boat, so as to reduce the friction.
However, since axles and bearings (e.g. plates with through-holes) for the wheels of the above-mentioned quartz boat are made of quartz, the axles seize with the bearings so that the wheels cannot turn, when the quartz boat is heated at temperatures above 1000.degree. C. during the diffusion or oxidation process. Moreover, the quartz wheels seize with the quartz tube. In order to avoid such seizures, generally, the quartz boat is incessantly moved back and forth during the diffusion or oxidation process. However, the movement of the quartz boat causes the generation of the finely powdered quartz from friction surfaces, i.e., contacting surfaces between the axles and the bearings and between the wheels and the quartz tube. When the wheels, axles and bearings of the above-mentioned polycrystalline silicon boat are made of polycrystalline silicon, and the boat is used for heat-treatment in an oxidizing atmosphere such as oxygen or water vapor, the surfaces of the wheels, axles and bearings are converted into silicon dioxide and seizure occurs in the same manner as the above-mentioned case of the quartz boat.